Kinetics and modeling of Pb (II) adsorption in pellet biochar based on micro-computed tomography characterization

Biochar, a cost-effective adsorbent for the removal of heavy metals from aqueous solutions, has gained increasing attention. In this study, an advanced micro-computed tomography (micro-CT) system was used to investigate the adsorption kinetics by direct localization and visualization of Pb (II) on wheat straw pellet biochar. The normalized digital images indicating the dynamic changes of Pb (II) adsorption on biochar samples at different initial Pb (II) concentrations of 100, 200, 300, and 400 mg/L and adsorption times were obtained. It was found that image grayscale (GS) changes over adsorption time (t) followed the power function, GS_e/GS_t=2.45∗t^-0.27. Based on this finding, modified pseudo-first-order (PFO) and pseudo-second-order (PSO) models incorporated with time-dependent kinetic constants k_PFOt=K_PFO∗GS_e/GS_t and k_PSOt=K_PSO∗GS_e/GS_t were proposed, resulting in a better interpretation of the adsorption mechanism. The micro-CT-guided novel approach demonstrated visual evidence-based superiority and should prove valuable to the existing body of research in related fields.